1. Field of the Invention
The present invention relates to reduction-resistant dielectric ceramic compacts and laminated ceramic capacitors comprising dielectric ceramic layers formed of the reduction-resistant dielectric ceramic compacts, and more particularly, relates to a laminated ceramic capacitor which is advantageously used in a high-frequency AC region or in an intermediate to high DC voltage range and which comprises internal electrodes formed of a base metal, and to a reduction-resistant dielectric ceramic compact for forming dielectric ceramic layers for use in the laminated ceramic capacitor.
2. Description of the Related Art
Heretofore, laminated ceramic capacitors are generally manufactured in a manner as described below.
First, ceramic green sheets, which contain a dielectric material and are coated with an electrode material for forming internal electrodes, are prepared for forming dielectric ceramic layers. As the dielectric material, a material primarily composed of, for example, BaTiO3 is used. Next, the ceramic green sheets coated with this electrode material are laminated to each other and are then bonded together by thermo-compression bonding, and the laminate thus formed is fired, thereby yielding a ceramic laminate having the internal electrodes. Subsequently, external electrodes, which are electrically connected to the internal electrodes, are provided on end surfaces of this ceramic laminate by firing, whereby a laminated ceramic capacitor is obtained.
Consequently, a material which is not oxidized during firing of a ceramic laminate has been generally selected as the material used for the internal electrodes. For example, noble metals, such as platinum, gold, palladium and a silver-palladium alloy, have been used as the materials for the internal electrodes. However, even though these internal electrode materials have superior characteristics, since they are significantly expensive, these materials are most responsible for an increase in manufacturing cost of the laminated ceramic capacitors.
Accordingly, a laminated ceramic capacitor has been proposed which uses a relatively inexpensive base metal such as nickel or copper as the internal electrode material in order to reduce the manufacturing cost.
However, these base metals mentioned above are easily oxidized at a high temperature in an oxidizing atmosphere, and as a result, they cannot serve as the internal electrodes. In order to use a base metal as the internal electrodes for the laminated ceramic capacitor, firing for obtaining the laminated ceramic capacitor must be performed in a neutral or a reducing atmosphere.
In addition, when firing is performed at a low partial pressure of oxygen in the neutral or the reducing atmosphere described above, the ceramic compact for forming dielectric ceramic layers is significantly reduced, and as a result, a problem may occur in that the ceramic compact starts to have semiconductor characteristics.
Accordingly, as a reduction-resistant dielectric ceramic compact which is not likely to have semiconductor characteristics even though fired at a low partial pressure of oxygen for preventing oxidation of a base metal, for example, a BaTiO3-(Mg,Zn,Sr,Ca)O-B2O3-SiO2-based dielectric ceramic compact disclosed in Japanese Examined Patent Application Publication No. 61-14611, a (Ba,M,L)(Ti,R)O3-based dielectric ceramic compact (in which M is Mg or Zn, L is Ca or Sr, and R is Sc, Y, or a rare earth element) disclosed in Japanese Unexamined Patent Application Publication No. 7-272971, and the like have been proposed.
Concomitant with trends toward higher integration, improved performance and lower price of electronic devices, laminated ceramic capacitors are increasingly subject to more adverse usage conditions, and hence, lower loss, improved insulating characteristics, improved breakdown voltages, improved reliability, larger capacity, lower price and the like are strongly required for the laminated ceramic capacitors.
In addition, laminated ceramic capacitors which can be used under high frequency conditions of high voltage or large current are increasingly in demand in recent years. The important properties required for these laminated ceramic capacitor are low loss and low heat generation. The reason for this is that when the loss and heat generation are large, the life of the laminated ceramic capacitor itself is decreased. Furthermore, due to the loss and the heat generation of the laminated ceramic capacitor, an increase in temperature occurs in the circuit containing them, and as a result, malfunctions of peripheral units and a decrease in life thereof also occur.
The laminated ceramic capacitors are also increasingly used under high DC voltage conditions. However, particularly in conventional laminated ceramic capacitors using nickel as an internal electrode material which are designed to be used under relatively low electric field conditions, when used under high electric field conditions, the insulating characteristics, breakdown voltage and reliability are degraded.
When a laminated ceramic capacitor is formed by using the dielectric ceramic compact disclosed in Japanese Examined Patent Application Publication No. 61-14611 or Japanese Unexamined Patent Application Publication No. 7-272971, even though the rate of change in static capacitance with temperature is not significant, there are shortcomings in that the loss and the heat generation are significant when used under high frequency conditions of high voltage or large current. In addition, since the dielectric ceramic compact described above is reduction-resistant, a base metal such as nickel can be used as the internal electrode material when firing at a low partial pressure of oxygen is performed; however, the firing at a low partial pressure of oxygen is hard firing for the dielectric ceramic compact, and for example, when an obtained laminated ceramic capacitor is used under high DC voltage conditions, there are shortcomings in that the insulating resistance is low and that the reliability is poor.
Accordingly, an object of the present invention is to provide a reduction-resistant dielectric ceramic compact for advantageously forming dielectric ceramic layers for use in, for example, a laminated ceramic capacitor, which has a low loss and low heat generation when used under high frequency conditions of high voltage or large current, and which exhibits a stable insulating resistance under AC or DC high temperature loading conditions.
Another object of the present invention is to provide, in addition to the object described above, a laminated ceramic capacitor which can use a base metal such as nickel or a nickel ally as an internal electrode material.
A reduction-resistant dielectric ceramic compact of the present invention comprises an auxiliary sintering agent and a solid solution comprising a barium titanate-based perovskite compound represented by the formula ABO3 as a primary component.
In the reduction-resistant dielectric ceramic compact, the crystalline axis ratio c/a obtained by x-ray diffraction in a temperature range of xe2x88x9225xc2x0 C. or above satisfies 1.000xe2x89xa6c/axe2x89xa61.003, and the maximum peak for temperature dependence of the dielectric constant measured at an electric strength of 2 Vrms/mm or less and at an AC frequency of 1 kHz is present at a temperature of below xe2x88x9225xc2x0 C.
The primary component described above is represented by the formula ABO3+aR+bM.
In this formula described above, R is a compound containing at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, M is a compound containing at least one element selected from the group consisting of Mn, Ni, Mg, Fe, Al, Cr and Zn, and a and b indicate the number of moles of the compounds mentioned above in the chemical formula each containing one element among the elements mentioned above.
In addition, in the formula described above, preferably 1.000 less than A/Bxe2x89xa61.035, 0.005xe2x89xa6axe2x89xa60.12, and 0.005xe2x89xa6bxe2x89xa60.12.
In the reduction-resistant dielectric ceramic compact of the present invention, about 0.2 to 4.0 parts by weight of the auxiliary sintering agent is preferably present with respect to 100 parts by weight of the primary component.
In addition, the primary component preferably comprises X(Zr,Hf)O3 in the reduction-resistant dielectric ceramic compact of the present invention, in which X is at least one element selected from the group consisting of Ba, Sr and Ca. X(Zr,Hf)O3 can range of from zero to about 0.20 mole with respect to 1 mole of ABO3 in the primary component.
In the reduction-resistant dielectric ceramic compact of the present invention, the primary component preferably comprises D which is a compound containing at least one element selected from the group consisting of V, Nb, Ta, Mo, W, Y, Sc, P, Al, and Fe. In the case described above, D in the range of from zero to 0.02 mole is more preferably contained with respect to 1 mole of ABO3 in the primary component.
In the reduction-resistant dielectric ceramic compact of the present invention, the primary component may comprise X(Zr,Hf)O3 and D. With respect to 1 mole of ABO3 in the primary component, it is preferable that X(Zr,Hf)O3 be in the range of from zero to about 0.20 mole and the D be contained in the range of from zero to about 0.02 mole.
In the reduction-resistant dielectric ceramic compact of the present invention, when the barium titanate-based perovskite compound represented by ABO3 is represented by the chemical formula {(Ba1-x-ySrxCay)O}mTiO2, x, y and m preferably satisfy 0xe2x89xa6x+yxe2x89xa60.20 and 1.000 less than mxe2x89xa61.035, and with respect to 100 parts by weight of the barium titanate-based perovskite compound, it is preferable that compounds comprising at least one element selected from the group consisting of S, Na and K be in the range of about 0.5 part by weight or less calculated as SO3, Na2O and K2O, respectively, and comprising Cl be in the range of about 5 parts by weight or less.
In the reduction-resistant dielectric ceramic compact of the present invention, the auxiliary sintering agent preferably comprises a compound containing boron, a compound containing silicon and a compound containing boron and silicon. In particular, the compound containing silicon is preferably silicon oxide.
The present invention may be applied to a laminated ceramic capacitor comprising a plurality of dielectric ceramic layers, internal electrodes formed between the dielectric ceramic layers and external electrodes electrically connected to the internal electrodes. In the laminated ceramic capacitor described above, the dielectric ceramic layers comprise the reduction-resistant dielectric ceramic compact according to the present invention described above.
In the laminated ceramic capacitor of the present invention, the internal electrodes may be formed of nickel, a nickel alloy, copper or a copper alloy.
In addition, in the laminated ceramic capacitor of the present invention, the external electrodes may each comprise a first layer composed of a sintered layer containing a powdered conductive metal or of a sintered layer containing a powdered conductive metal and one of a glass frit, a crystallized glass and a ceramic; and a second layer which is disposed on the first layer and which is a plating layer.